As Ruben mentioned yesterday, high pressure centered over the Eastern U.S. is keeping temperatures in the 90s °F and dew points in 60-70 °F range across the region. These conditions are allowing haze and associated Code Yellow (Moderate) PM2.5 to persist across much of the Mid-Atlantic and Southeast. The haze is evident in today's MODIS true color image (below on upper left, overlaid with AQI levels and the synoptic analysis) and AOD (below on upper right). At the surface, the haze isn't too thick today, however; a webcam image from this afternoon at McMillan reservoir (below on lower left, courtesy of the National Park Service) shows a relatively clear view of the National Mall. PM2.5 concentrations in the DC/Baltimore region are in the lower Code Yellow range, averaging around 16-20 ug/m3 (below on lower right, courtesy of AIRNow-Tech).
Clear skies, light winds, and long days with a high solar zenith angle are ideal conditions for ozone generation, and as a result, many states in the Mid-Atlantic, Mississippi Valley, and Southern California issued Ozone Air Quality Action Days for today. But by 5:00 PM EDT, only scattered area in the Mid-Atlantic and Southern California were experiencing Code Orange (Unhealthy for Sensitive Groups) ozone levels (below on left). With another day of high temperatures and sunny skies expected tomorrow, a range of metropolitan regions across the country have issued Ozone Air Quality Actions Days for Thursday, including parts of the Mid-Atlantic, Southeast, Texas, Utah, and the Pacific Southwest (AQI forecasts below on right in Google Earth).